So called “epoxy resins” are polymeric materials characterized by the presence of more than one epoxide ring per molecule on average. Epoxies have a tremendous range of applications in modern day society. These applications include coatings on metal cans, automotive primers, printed circuit boards, semiconductor encapsulants, adhesives, and aerospace composites. Epoxies are, in general, thermoset polymers which, in the presence of a curing agent, or cross-linking agent, undergo extensive cross-linking to form a three dimensional polymer network. Most cured epoxy resins form relatively dimensionally stable amorphous networks with excellent mechanical strength and toughness; outstanding chemical, moisture, and corrosion resistance; and variously good thermal, adhesive, and electrical properties. The highly useful combination of properties, along with versatility in formulation and low cost, have led to the widespread use of epoxies in a plethora of adhesive, structural, and coatings applications.
Great demands are placed upon the epoxy employed for the built up layers in the multilayer printed circuit board application. The dielectric layer is desirably matched in coefficient of thermal expansion (CTE) to that of the silicon chip (3-4 ppm/° C.), to avoid the creation of misalignments during thermal cycling, and to that of the copper (ca. 17 ppm/° C.) to avoid stress-induced delamination. Recently, CTE in the out-of-plane direction has taken on increasing importance. The uncured composition must be of sufficiently low viscosity at the lamination temperature so that it will fill all the space between conductors; and the cured epoxy must retain sufficient toughness (usually measured as elongation to break) to endure being dropped in use (as in, e.g., a cellular phone). It is also desirable that a simple chemical etching of the surface to provide the desired degree of roughness—ca. 0.1 micrometer—for copper deposition by electroless and electrolytic methods which will maintain an adhesive strength of ca. 6 N/cm.
Most common dielectrics layers are epoxy resins which exhibit CTE values of 70 ppm/° C. and higher. One approach to achieving reduction of CTE has been to employ heavy loadings of inorganic fillers on the order of 1-20 micrometers in average particle size. While reducing CTE, high loadings of inorganic fillers have resulted in increased brittleness, increased viscosity, poor adhesion strength, and degraded dielectric properties.
There is a clear need in the art for improved property trade-offs in epoxies employed in building up multilayer printed circuits.
Nakamura, US Patent Publication 2005/0129895 discloses the use of 30% by weight of spherical silica of unstated size in an epoxy composition. Spherical silica is available commercially in particle sizes from 0.3 micrometer and larger. Nakamura provides no determinations of viscosity or flow performance, no measurement of CTE, no determination of impact toughness.
Yasu, Japanese unexamined patent application JP 02263858 (abstract only), discloses epoxy compositions comprising ca. 60% by weight of hydrated alumina. No information about viscosity or flow is provided; no information regarding impact toughness, nor adhesive strength is provided.
Cochrane et al., Modern Paint and Coatings (October, 1983), discloses incorporation of a mixture of fumed silicas differing in particle size into epoxy compositions for the purposes of rheology control. Loadings of silica are 5% by weight or less.